1. Field of the Invention
The present invention relates to a focus state detection device, and more particularly, to a focus state detection device adapted for use in a camera and capable of detecting focus condition in a plurality of focus detecting areas.
2. Description of Related Art
A known focus state detection device is capable of detecting focus condition in a plurality of focus detecting areas. Focus condition can be detected simultaneously not only in the area around the optical axis of the photographic lens but also in areas off the optical axis. Such a device is a modification of a known optical system used for detecting focus condition that detects focus condition in only one area. Only the number of areas is increased.
The difficulties created with such a system and the method of resolving such difficulties are discussed in Japanese Publication No. 63-289513, corresponding to U.S. Pat. No. 4,857,718. FIG. 8 corresponds to an embodiment of a device described in the 718 Patent. The device detects focus condition in a total of three locations, namely, in an area on the optical axis of the photographic lens and in two areas not on the optical axis. The positioning of these areas, as indicated on focal plane FP of photographic lens TL, is such that areas E2 and E3 extend in the vertical direction and are not on the optical axis. Areas E2 and E3 surround area E1, which extends in the horizontal direction and is on the optical axis.
In each of these areas, two light rays (LW1, LW2), (LW3, LW4) and (LW5, LW6) pass through different regions of photographic lens TL, pass through field lenses L01-L03, and are composed as images on light-receiving areas (P1,P2), (P3,P4) and (P5,P6) of CCD line sensor P0 by three pairs of re-imaging lenses (L1,L2), (L3,L4) and (L5,L6). The focus position of photographic lens TL in each area is detected from the position of the images.
With this type of plural focus detecting system, field lenses L01-L03 and re-imaging lenses (L1,L2), (L3,L4) and (L5,L6) normally are single entities (i.e., one-piece units), as shown in FIG. 8, to conserve space and materials.
Field mask FM has apertures E01-E03 that restrict the range of the area where focus condition is detected. Field mask FM is immediately in front of field lens L0, which comprises three lenses L01-L03 formed as a single entity. Mask AM has apertures (A1,A2), (A3,A4) and (A5,A6), which restrict the range where the light rays used for focus detecting pass through the exit pupil of photographic lens TL after being projected to the exit pupil by the field lenses L01-L03. Mask AM is placed in front of re-imaging lens L, which comprises the three pairs of lenses (L1,L2), (L3,L4) and (L5,L6) formed as a single entity. The focus state detection optical system in one of the areas, such as the area E1 on the optical axis, for example, comprises aperture E01 in field mask FM, field lens L01, a pair of apertures (A1,A2) in mask AM, and a pair of light-receiving areas (P1,P2) of CCD line sensor P0. The number of focus state detection optical systems agrees with the number of focus detecting areas.
With focus state detection devices equipped with a plurality of focus state detection optical systems, the light rays used for each focus detection become closer to each other as CCD line sensor P0 is made smaller, that is, as distance Q in FIG. 8 is made smaller, to make the device more compact. A problem arises in that light rays from areas not corresponding to a particular light-receiving area can be incident upon that light-receiving area in the CCD line sensor. This harmful stray light, indicated in FIG. 8 by dashed lines LWh1 and LWh2, causes the precision of focus state detection to drop.
To solve this problem, in U.S. Pat. No. 4,857,718, a blocking material MM has three apertures M1-M3, through which light rays for focus detecting can pass. Blocking material MM is positioned between field lens L0 and re-imaging lens L to block stray light rays.
FIG. 7 shows positioning of stray light-blocking material MM of U.S. Pat. No. 4,857,718 with a focus state detection device mounted in a camera. Deflecting mirror M is between field lens L0 and re-imaging lens L so that the camera is not excessively large. Light rays for focus detecting are deflected nearly perpendicular to the direction of the photographic lens, to efficiently use space in the camera. In U.S. Pat. No. 4,857,718, blocking material MM is either positioned to be perpendicular to the optical axis of the light rays for focus detecting at position h, between re-imaging lens L and bending mirror M, or is adhered to the top surface of deflecting mirror M.
Additionally, a focus state detection device capable of focus detecting in plural focus detecting areas is disclosed in Japanese Publication No. Hei 4-422. Focus detecting areas not on the optical axis are arranged not only to the left and right of the axis, but also at the top of the screen, above the axis. This arrangement is shown in FIG. 4. In addition to central area a on the optical axis and areas b and c to the left and right of the optical axis, area d to the top and not on the optical axis also is provided. Field lens L0, re-imaging lens L and CCD line sensor P are formed so that all components corresponding to each area are comprised of a single entity.
The arrangement of the areas in the optical system is shown in FIG. 5. Area d is parallel to central area a on the optical axis, both areas extending horizontally on the screen. In addition, an area also is provided toward the bottom of the screen in Japanese Publication No. 62-189415, as shown in FIG. 6. In FIG. 6, bottom area e has been added.
Furthermore, a cross-type arrangement such as that shown in FIG. 2 can be used. Central area a on the optical axis, which has a high frequency of use, can be used to detect the focus condition of a subject extending parallel to either the vertical direction or the horizontal direction.
FIG. 3 is a cross-sectional view of a focus state detection optical device having the plurality of focus detecting areas shown in FIG. 2. In a detection state, that is, a viewfinder observation state, light rays that have passed through the photographic lens are divided by partially reflective main mirror M1 into light rays 21 for viewfinder observation and light rays 22-24 for focus detecting in each area. Light rays 21 for observation are reflected in the direction of focusing plate S by main mirror M1. Light rays 22-24 for focus detecting pass through main mirror M1 and are deflected toward bottom B of a mirror box by a completely reflective sub-mirror M2. Mirrors M1 and M2 each are supported by a respective mirror holder 1 and 2, and rotate around respective rotation shafts (not shown) during sequencing of the camera, to dodge photographic light rays. Opening 1--1 is provided in main mirror holder 1 to allow light rays 22-24 to pass to sub-mirror M2 from main mirror M1 for focus detecting.
The basic structure of this focus state detection optical system is generally the same as in FIG. 7. Light rays 22-24 for focus detecting pass through field mask 3 and field lens 5, are deflected toward the photographic lens by deflecting mirror M0, and pass through mask 6 and re-imaging lens 7 to reach CCD line sensor 8. In addition, infrared reduction filter 4 is directly in front of field lens 3 to correct the relative luminosity factor of CCD line sensor 8. Each of these components in the optical system is fixed to detecting focus condition holder 9, and together comprise a detecting focus condition module.
As described above, areas not on the optical axis are provided above and below the optical axis. Consequently, three of the focus state detection optical systems corresponding to these areas are lined up along a cross-sectional plane that includes optical axis 20 of the photographic lens and the optical axes of light rays 22-24 for focus detecting. Field lens 5 comprises three component lens elements 5-1, 5-2 and 5-3, formed together as a single entity. Re-imaging lens 7 includes a pair of component lenses 7-2 and 7-3 in a direction perpendicular to the plane of FIG. 3, corresponding to areas not on the optical axis. These areas extend in the horizontal direction. Re-imaging lens 7 also includes two pairs of component lenses 7-1, corresponding to the central area on the optical axis. One pair is in a direction parallel to the plane of FIG. 3 and one pair is in a direction perpendicular to the plane. The overall area is in the shape of a cross, and all of the lenses are formed together as a single entity.
Stray light harmful to focus detecting is produced in this type of detecting focus condition optical system capable of focus detecting in a plurality of focus detecting areas. For light passing through the apertures of the field mask for the upper and lower areas, stray light passes through field lens 5-1 at the central area from the surface on the side opposite the field mask, as indicated by dashed lines 25 and 26.
Stray light 26 passes through aperture 3--3 of the field mask for the upper area, is incident on field lens 5-3 from the field mask side of the lens, and exits from the exit pupil surface of field lens 5-1 on the side opposite the field mask. Stray light 26 is dispersed by tip M2a of sub-mirror M2 and enters the gap between the tip and shutter T. However, if tip 2a of sub-mirror holder 2 is lengthened relative to sub-mirror M2, as shown in FIG. 3, thereby making the spacing from shutter T extremely small, it is possible to block nearly all stray light 26 incident on field lens 5-3. Accordingly, stray light 26 can be blocked prior to being incident on the focus state detection module.
Stray light 25 passes through aperture 3-2 of the field mask for the lower area, is incident on field lens 5-2 from the field mask side, and exits from the exit pupil surface of field lens 5-1 on the side opposite the field mask. Stray light 25 comprises light that strays toward main mirror M1 from tip M2b of sub-mirror M2 and toward the photographic lens, and which passes through opening 1--1 in main mirror holder I after passing through the partially-reflective main mirror. The light includes light rays of reverse incident light from the viewfinder eyepiece, which pass through nearly the center of the focusing plate S, where reverse incident light is the strongest. Moreover, the areas through which the reverse incident light passes, including both the partially reflective area of main mirror M1 and opening 1--1 in main mirror holder 1, also are areas through which light rays 23 for focus detecting pass. It thus is impossible to block reverse incident light at the main mirror or at the focusing plate.
In addition, stray light 25 overlaps with light rays 22-24 used for focus detecting, which are shown in solid lines, between field lens 5 and re-imaging lens 3. Blocking stray light at this position thus also would block the light used for focus detecting. It is impossible, therefore, to block only stray light with the light-blocking plate of U.S. Pat. No. 4,857,718. Overlap between stray light and the light used for focus detecting in the central area on the optical axis occurs more easily when the central area on the optical axis is in the shape of a cross. Overlap also occurs more easily for-large widths of focus detecting light rays 22 in the central area on the optical axis in the plane of FIG. 3.
Stray light 25 is produced by a plurality of field lenses formed as a single entity for the different detecting focus condition blocks. Light incident on the field lens from the field mask side enters a region of the field lens of a focus state detection block corresponding to an area adjacent to the area the light was incident on within the field lens. The light emerges from the exit pupil surface of a field lens corresponding to an area different from the area in which the light entered.